One of the TOP ten government contractor applied for DHS Public trust clearance for me. It took about 5 months for approval. By that time clearance being approved, I joined another job which does not required clearance. So I declined the job. I'm thinking to go back as government contractor again. My clearance was approved two months ago, I'm wondering whether I can use that clearance to join another company as a contractor.
From what I can gather here and here, security clearances aren't tied to you as an individual, they're tied to your role at a particular employer (where said role warrants/requires a security clearance). There's a small caveat to that if you're an independent contractor who's working on/bidding for government contracts, however even then your clearance is only technically granted within the scope of a particular project (or set of projects).
On that basis, I'd say the answer is no, you cannot reuse the clearance. That's something that was issued in the context of you performing some specific role for a specific government contractor, and it doesn't carry over to a new employer.
However, you should be able to reuse any information that was collected as part of the background investigation that was used to determine whether or not to grant your security clearance originally, provided the information itself hasn't expired yet. That may make getting approved for clearance with the new employer faster/easier than the initial process.